I split the cases on my viper engine today, (there's a thread about it in the tech questions section) and of course had to take the pistons off...and I noticed some interesting things. Both the pto and center pistons look different than the used stock piston I got from mrviper700. They're coated with something, so I guess they're aftermarket pistons? Also the center piston has 2 notches in the bottom of the skirt, anyone seen anything like this before? Is this ok? I guess the engine's been rebuilt before, and when I bought it there was only 900 mi on it. Will I be able to put the stock piston in with those two?
if you look inside the piston you got from me youll see 8EK, thats a yamaha powder forged stock piston, the good ones, the other pistons you have are those cheap cast ones from ebay I am guessing, look on bottom do they say 8ek? If not, I would not mix them due to the weight will most liely be differnt, you can weigh them on a gram scale to be sure, then post results, but those other 2 are not stockers most likely, unless someone had them swain tech coated, could be? youll know as soon as you look inside. the notchs at the bottom would achieve absolutley nothing!
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They're definitely aftermarket..they don't say 8EK. The piston that burned up was also aftermarket. I guess I need two more stock pistons, then three new sets of rings...though I should be able to use the pins and bearings right? The bearings seem fine to me. Do you have another two used ones by chance? Thanks.
I gotta look for you, very possible have 2 more. Yes you could use the pins and bearings as long as the pins are the same thickness and weight. Wont be able to check till Monday, going bambi hunting in morning be gone all day.
redsnake3
New member
those are spx pistons, they have raised lines above the top ring to the top of the piston, had a set for my viper and they weight like 10 grams + less than a stock piston. but how come the stock viper piston doesnt have the transfer port hole?
it does, its just backwards in the pic, thats why you cant see the ring locator pins, thats the exh side you see.
RTY, just make sure that they are the same size pistons...mic the cylinder bore then the pistons...most likely if someone replace all the pistons, it might have been out of size & they went 10ths over size to make up for worn bores in the cylinder...longer skirts are common in oversize after market pistons, helps in stabilizing it in the cylinders...notches are there to clear the crank counter weights...It will be challenging now for you to rebuild this...you have some miss match stuff, just do your homework, be patient & mic everything & record the values from mag to pto...bores of each cylinder w/each piston...you have a manual, then start with the stock measurements (spots & places to take measurements from...Oh mark each spot with a black felt marker to as reference points) see what your starting point is & what it is now at...when you assemble everything up, before putting on the head...do a top dead centre to check piston heights & then do a squish test with the head on after...I put plasticine on the end of my spark plugs to see if the pistons comes in contact that plug, then do the other one & so on...If after everything is good, no clearance issues...only then would I install it back in the sled...good luck, keep us posted...If the whole project is a little overwhelming, than just send it out to mrviper or cb, parts are costly enough & if you got to do it twice then your no better off...if you know what I mean...cheers
theblues said:
I'll check the size of the pistons and cylinder bore, and yes I do have a service manual and will do all those measurements, but I do know that the stock piston is the exact same height and the pins are at the same height as the aftermarkets. It's a mystery to me as to why those notches are there on only the center piston. If it's the same height as the stock piston, it shouldn't need to clear anything. Wouldn't that throw everything out of balance too, or isn't that enough? Lining up the skirts of the stock and aftermarket, they appeared to be the same diameter, not oversize. I don't have a micrometer here now but I can use one tomorrow to check for sure. Also, the sled only had 900 miles on it, so the cylinder bores should not be worn to the extent of needing oversize pistons, though I guess it is possible and I won't know until I mic them. If the cylinders ARE within spec, then I should be able to install stock yamaha pistons and pins and be ok right? I'm thinking that someone didn't want to pay the price for yamaha pistons so they went with a cheap set. I was thinking of re-using the wrist pin bearings, because they appear to be in good shape (they should be with only that amount of miles on them) though I'll weigh them on a gram scale to make sure they're the same as the yamaha.
What do you mean by a squish test? Is that checking the piston/plug clearance with the head torqued?
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Here's ya go, I use solder core, the type for plumbing...
Squish Test
Cut soft lead solder square on the end with a utility knife and bend into an "L" shape. Put the solder thru the spark plug hole, align with the centerline of the crankshaft, and push end tight against cylinder wall over the wrist pin in the piston. Turn the engine over very slowly with the clutch as you reach TDC. Remove, cut off flattened area, record thickness with a vernier, and repeat on the other side of piston. Mark PTO & MAG for each cylinder and M & P for each side of cylinder. Masking tape will hold your pieces in order for future reference. Solder cut square and put tight against the cylinder wall usually squishes down beside the piston - proof you got and accurate measurement of piston to head clearance.
PTO Cylinder CEN Cylinder MAG Cylinder
P = ________ M = ________ P = ________ M = ________ P = ________ M = ________
Inch or MM? Inch or MM? Inch or MM?
Squish Test
Cut soft lead solder square on the end with a utility knife and bend into an "L" shape. Put the solder thru the spark plug hole, align with the centerline of the crankshaft, and push end tight against cylinder wall over the wrist pin in the piston. Turn the engine over very slowly with the clutch as you reach TDC. Remove, cut off flattened area, record thickness with a vernier, and repeat on the other side of piston. Mark PTO & MAG for each cylinder and M & P for each side of cylinder. Masking tape will hold your pieces in order for future reference. Solder cut square and put tight against the cylinder wall usually squishes down beside the piston - proof you got and accurate measurement of piston to head clearance.
PTO Cylinder CEN Cylinder MAG Cylinder
P = ________ M = ________ P = ________ M = ________ P = ________ M = ________
Inch or MM? Inch or MM? Inch or MM?